1. Field of the Invention
The present invention relates to a thin film transistor of a liquid crystal display panel, and more particularly, to a thin film transistor capable of increasing ON current and decreasing OFF current values, a thin film transistor substrate having the same, and a method of fabricating the same.
2. Description of the Related Art
A liquid crystal display (LCD) apparatus typically displays an image via liquid crystal cells arranged in a matrix format on a liquid crystal display panel controlling light transmissivity in response to a video signal. Each of the liquid crystal cells uses a thin film transistor (TFT) as a switching element for independently providing a video signal. The TFT uses an amorphous silicon layer or a polysilicon layer as its active layer. When a polysilicon layer is used as the active layer of the TFT, a driving circuit requiring a high response speed can be used for the liquid crystal display panel because electric charge mobility of polysilicon is approximately a hundred times greater than that of amorphous silicon.
FIG. 1 roughly illustrates a conventional LCD panel using a polysilicon TFT. Referring to FIG. 1, the LCD panel includes an image display part 96 including a liquid crystal cell matrix, a data driver 92 for driving data lines 4 of the image display part 96, and a gate driver 94 for driving gate lines 2 of the image display part 96.
The image display part 96 has liquid crystal cells LC arranged in a matrix format to display images. Each of the liquid crystal cells LC has a TFT 30 connected to a corresponding gate line 2 and data line 4. The TFT 30 charges a video signal from the data line 4 in the liquid crystal cell LC in response to a scan signal of the gate line 2. The liquid crystal cell controls light transmissivity in a manner such that liquid crystals having dielectric anisotropy respond to the charged video signal for representing gray scales.
The gate driver 94 sequentially drives the gate lines 2. Moreover, the data driver 92 provides a video signal to the data lines 4 when the gate lines 2 are driven.
The above-described LCD panel is constructed in such a manner that a TFT substrate on which the TFT 30 of the liquid crystal cell LC, the data driver 92 and gate driver 94 are formed is bonded to a color filter substrate on which a common electrode and a color filter are formed, with a liquid crystal layer being disposed therebetween.
A conventional TFT is decreased in size as the resolution of the LCD panel employing the TFT is increased. However, there are certain difficulties associated with conventional TFT's such as when the size of a conventional TFT is reduced, the channel length of the TFT is also decreased, resulting in the reliability of the TFT being deteriorated as well. For example, with conventional TFT's, a hot carrier stress in a gate insulating layer is generated when electrons are accelerated from a source region to a drain region, thereby reducing electron mobility. Further, a depletion region is formed in the vicinity of the drain region of the active layer due to an electric field formed between gate and drain electrodes so as to change an energy band within the LCD apparatus. Moreover, the above depletion region further increases as the electric field between the gate and drain electrodes increases, thereby resulting in a high electric field being applied between the gate and drain electrodes to generate gate induced drain leakage (GIDL). GIDL refers to the occurrence of an increase in current caused by band tunneling of electrons.
Due to the above described GIDL and hot carrier stress, a conventional polysilicon TFT substrate generates a typical short channel effect that decreases a source/drain breakdown voltage and a threshold voltage and also increases swing of the LCD apparatus.